man: remove note about shutdown being legacy

This can only confuse people, because there's no need to
discourage people from using shutdown. It is fully functional
and supported.
man: use <literal> not <filename> for suffixes

Especially sentences like "filename ends in .suffix" are easier to
parse if the suffix is surrounded by quotes. In sentences like
"requires a .service unit", where the suffix is used as a class
designation, there is no need to use quotes.
man: document the slice and scope units, add systemd.cgroup(5)